The present invention relates generally to propellant compositions which produce a minimum amount of smoke. Propellants are chemical compounds or mixtures thereof which, upon ignition, generate large volumes of hot gases at controlled, predetermined rates. Propellants serve as a convenient, compact form of storing relatively large amounts of energy for rapid release and enjoy utility in various industrial and military applications. Thus, propellants are generally employed in various situations requiring a readily controllable source of energy, as for ballistic applications, e.g., for periods of time ranging from milliseconds in weapons to seconds in rocketry, wherein the generated gases function as a working fluid for propelling projectiles such as rockets and missile systems.
In use, a propellant grain is typically placed within the interior of the case of a rocket motor. The propellant forming the grain is combusted to provide a thrust within the interior of the rocket motor case. The rocket motor derives its propellant thrust from the formation of the hot generated gases through the throat and nozzle of the motor case. Solid propellants are also employed extensively in the aerospace industry. Solid propellants have developed as the preferred method of powering most missiles and rockets for military, commercial, and space applications, because they are relatively simple and economic to manufacture and use, and they have excellent performance characteristics and are very reliable.
Different propellant applications, however, may impose a peculiar requirement on the propellant composition linked to a particular utility. There are several applications in which the rocket motor is required to perform with minimal or no smoke output. For example, in tactical rocket motors, the production of smoke is disadvantageous, particularly in shoulder-launched rockets, wherein generated smoke may obscure the user's vision and toxic components entrained in the smoke may even cause short and/or long-term adverse effects, such as eye damage. In addition, tactical rockets launched from an aircraft or vehicle will also require minimal or no generated smoke which may obscure the vision of a pilot or vehicle operator. Moreover, the production of smoke facilitates tracking the source of the launched rocket by enemy forces, particularly when used in an anti-tank capacity, a serious disadvantage during military operations.
An important consideration in solid propellants, including minimum smoke propellants, is the provision of satisfactory energy output and burn rate of the propellant, without significantly adding to the smoke output of the propellant. It is important that the amount of energy delivered meet system performance requirements and space available, and that the propellant burn at a controlled and predictable rate. If a satisfactory burn rate of the propellant can be obtained, it is possible to assure proper operation of the rocket motor, or other similar device. If the propellant achieves an excessively high burn rate, the pressure created within the casing may exceed the design capability of the casing, resulting in damage or destruction to the device. If the propellant does not develop a sufficient burn rate, there may not be sufficient thrust to propel the rocket motor over the desired course.
In addition to energy and burning rate considerations, a propellant must meet other criteria including mechanical characteristics, stability, sensitivity, cost of manufacture, and uniformity of performance for optimal effectiveness. Other factors affecting propellant selection for guns and rockets, include manufacturing characteristics, such as the availability and cost of raw materials and processing equipment, simplicity and cost of manufacture and inspection, manufacturing hazards, and propellant viscosity and flowability; energy delivery requirements, such as specific impulse or force, loading density in terms of required burning characteristics, metal parts requirements in terms of operating pressure over a required temperature range; temperature dependance such as ignition, pressure, burning rate and thrust characteristics over temperature range; mechanical characteristics over temperature range; effect of high-low temperature cycling; reliability of performance including lot-to-lot variations in burning rate and pressure, effect of small variations in metal parts on performance, and effect of small variations in composition and dimensions on performance; long-term storage characteristics such as deformation changes, performance changes, moisture absorption, and exudation or migration of plasticizer; effects of mechanical characteristics, such as long-term storage, high-low temperature cycling, acceleration forces, rough handling and case bonding; compatibility with process equipment, with personnel (toxicity), with metal and plastic parts and other components, of reaction products with personnel, metal parts, and electronic equipment and erosive effects of reaction products; and system requirements such as smokeless exhaust, combustion stability, effect of exhaust plume on radar, absence of ignition peaks or reinforcing pressure waves, minimum gun smoke, flash and blast pressure, and detonation free in event of malfunction.
Another significant concern in the formulation of propellants is safety, because propellants are often employed or stored in an area in which other military ordinance is stored, and employed in environments which are conductive to accidental ignition, e.g., stray bullets or flying debris. Moreover, propellants must be formulated to avoid premature ignition by virtue of exposure to hot environments or under normal operating conditions. Thus, an important factor in formulating a propellant is insensitivity to premature or accidental ignition.
In addition, rocket propellants desirably exhibit adequate mechanical properties to withstand the stresses imposed during handling and firing. In many situations, rocket propellants must be capable of performing satisfactorily after undergoing thermal stresses produced during long-term exposure and cycling at extreme temperatures. In view of the recognized criticality of failure of a single grain in a rocket, rocket grains are subjected to a large number of tests and inspections to ensure that they satisfy certain minimum mechanical and physical characteristics. Well-established laboratory methods determine the tensile strengths, the modulus in tension and compression, elongation under tension, and deformation under compression of rocket propellants.
It has been found extremely difficult to formulate an effective rocket propellant which, upon combustion, generates a minimum or no amount of smoke and attendant particles while at the same time satisfies other requisite properties such as energy output, burn rate, insensitivity to accidental or premature ignition, ability to withstand long term storage and environmental stresses while meeting the broad range of military, industrial and research requirements. For example, ammonium perchlorate, a conventional oxidizer, cannot be used in minimum smoke propellant compositions because its presence results in the production of noxious gases that are toxic in man-rated environments. Moreover, propellant compositions are typically compacted into the form of grains of a suitable shape. Such propellant grains must be capable of sustaining thermal and tensile shock during igniter functioning, and must exhibit sufficient strength to remain intact during gas generator functioning if ballistic performance is to remain unaffected. The grains must retain such capability after aging and cycling.
Accordingly, there exists a continuing need for minimum smoke producing propellant compositions, particularly minimum smoke propellant compositions for man-rated, shoulder-launched rockets, which exhibit optimal ballistic properties.